Semiconductor devices comprise active and passive devices. The active devices can be, for example, a transistor or thyristor. A passive device can be, for example, resistors, diodes, capacitors or even wires. As to wiring fabrication, typically several layers of metal are used. These metal layers typically comprise a layered structure of aluminum, copper and/or TiN.
In fabricating the wirings, though, stresses can build up in the aluminum or other metal layer during annealing processes, which result in hillock formation. More specifically, during the annealing process, stress in the metal line can be relieved via vertical hillock formation as the grain pushes up to relieve the compressive stress. It has been found, though, that hillock formation can be suppressed by using a constraining layer, e.g., Si3N4, on top of the metal level. However, the use of this constraining layer results in the wires relieving stress via extrusion formation, rather than via hillock formation.
More specifically, extrusions in the aluminum layer are formed in response to constraints on the metal shapes that limit the ability of the metal to expand in response to an increase in temperature (as in an annealing step). The extrusions nucleate at locations where the metal microstructure is favorable and grow by self diffusion within the metal film. Hence, both the magnitude of the temperature increase and the duration of the exposure to the high temperature work to increase the size of the extrusion. Extrusions can be very problematic, in that they tend to grow towards neighboring lines. This growth can short devices, resulting in reliability and product yield concerns.